<?php
function add($a, $b){
    return $a - $b;
}
function get($name){
   return isset($_GET[$name])? $_GET[$name]:'';
}
function post($name){
  return isset($_POST[$name])? $_POST[$name]:'';
}
$a = post('a');
$b = post('b');
$result = add($a, $b);
?>
<form action="" method="post">
<input type="text" value=" <?php echo $a; ?>" name="a"/>-
<input  type="text" name="b" value="<?php echo $b; ?>"/>=<?php echo $result;?><br>
<input type="submit"/>
</form>
